The invention relates to a method and to a device for digitally coding binary data with a particular predetermined transmit signal spectrum on a communication channel, especially a serial data bus.
The data throughput and the data transmission rates on multiplexed communication buses are increasing more and more. By comparison, the quality of the communication channels used, for example of the data lines of a communication bus, has not significantly increased. Due to the high transmission rates, electromagnetic radiation occurs on the data lines so that the electromagnetic compatibility EMC decreases. To achieve high electromagnetic compatibility EMC, therefore, the data transmitted via the communication channel by a transmitting device are coded. The main coding methods used in this connection is the NRZ (Non-return to zero) coding method.
FIG. 1 diagrammatically shows the transmission of data from a data source DQ to a data sink DS via a communication channel K. The communication channel K is, for example, a data bus line of a serial data bus to which a number of controllers are connected which transmit and receive, respectively, data via transmitting devices S and receiving devices E, respectively. As an alternative, the communication channel K can also be a wireless transmission link as used in mobile radio.
FIG. 2 shows a typical interference signal spectrum of a communication channel K. In a low frequency range from 0 Hz up to a first frequency fa, the interference signal spectrum caused by interference signals has a relatively high level. The interference signals occurring in this range are mainly caused by poor ground contacts between the controllers and the communication channel or, respectively, the data transmission line. In a middle range of frequencies which extends from the first frequency fa up to a second frequency fb, the levels of the interference signals occurring are relatively low. Above an upper limit frequency fb, the levels of the interference signals increase considerably. In a vehicle system bus, for example, this is caused by high-frequency ignition interference signals.
The NRZ coding method performs data bit coding in the transmitting device S, in such a manner that the number of signal edges occurring in the coded signal is minimum. The result is that the coded signal transmitted in the communication channel K is below the upper limit frequency fb so that the transmitted coded signal will not be corrupted by any interference by high-frequency interference signals. However, the disadvantage of the NRZ coding method is that a long sequence of data bits which remain the same, for example a long sequence of data bits, output by the data source, all of which have a logical high state H, is transmitted via the communication channel by means of a coded transmit signal which has a very high direct-current signal component. The frequency of the coded signal transmitted is therefore below the lower limit frequency fa and is therefore exposed to a high-level interference spectrum.
A xe2x80x9cManchester coding methodxe2x80x9d has therefore been proposed in which a logical 0 output by the data source DQ is output as a transmit signal 01 by the transmitting device S and in which each logical 1 output by the data source DQ is transmitted as a coded transmit signal 10 from the transmitting device S to the receiving device E. Although the Manchester coding method has the advantage that the signal transmitted via the communication channel no longer has a direct-current component, a considerable disadvantage consists in that the coded signal transmitted via the communication channel K has a very high frequency signal component. For example, a constant sequence of logical zeros which are output by the data source DQ is transmitted as an alternating sequence of zeros and ones via the communication channel K in the Manchester coding method so that the coded signal transmitted has a very high frequency. The high-frequency signal components of the Manchester-coded transmission signal are disturbed by the high-frequency interference signals, the frequency of which is above the upper limit frequency fb, so that data transmission errors can occur.
It is, therefore, the object of the present invention to create a method and a device for digitally coding binary data in which the transmit signal spectrum of the coded transmit signal is in a frequency range having a minimum interference signal level.
According to the invention, this object is achieved by a method having the features specified in claim 1 and by a coding device having the features specified in claim 7.
The invention creates a method for digitally coding binary data with a particular transmit signal spectrum, the method exhibiting the following steps, namely
separating a digital data stream, which consists of the binary data to be coded, into data blocks having a predetermined data block length,
determining a difference between the number of binary data having a first binary state and the number of binary data having a second binary state, for each data block,
transmitting each data block as negated data block or as non-negated data block via a communication channel, in such a manner that the total sequence of data blocks transmitted on the communication channel has a particular transmit signal spectrum.
In the method according to the invention, preferably at least one indicating data item which specifies whether the transmitted data block is transmitted negated or non-negated is added to each data block transmitted.
In an especially preferred embodiment of the method according to the invention, the binary data are transmitted NRZ-coded.
The data blocks transmitted in the method according to the invention preferably in each case exhibit the same data block length.
In a preferred embodiment of the method according to the invention, the particular transmit signal spectrum exhibits a minimum direct-current component.
The transmit signal spectrum preferably additionally exhibits a minimum high-frequency signal component.
The invention also creates a coding device for coding binary data with a particular transmit signal spectrum, the coding device exhibiting
a data separating device for separating a data stream, which consists of the binary data to be coded, into data blocks having a predetermined data block length,
a calculating device for calculating the difference between the number of binary data having a first binary state and the number of binary data having a second binary state, for each data block, the coding device also having a transmitting device for transmitting each data block as a data block which has been transformed with respect to data or has not been transformed, via a communication channel connected to the transmitting device, in such a manner that the total sequence of the data blocks transmitted by the transmitting device on the communication channel exhibits the particular transmit signal spectrum.
The communication channel is preferably either a data transmission line or a wireless transmission signal link.